 These ratios for Uranium and Lead from Aeondae and Primordial Lead from Canyon Diablo are very important for dating meteorites. In the How Old is the Earth-Moon System, we covered radioactivity, half-life, and the law of radioactive decay. It showed that the present amount of radiogenic lead in a sample will equally initial amount plus however much gets created by the decay of the parent Uranium. For Lead 207 the parent is Uranium 235. For Lead 206 the parent is Uranium 238. With this equation we used Arthur Holmes' System for Uranium Decay into Lead inside Zircon crystals to date the oldest rocks on the Earth and the Moon. If that won't work for meteorites, they don't have any Zircon crystals. So by the mid-1940s Arthur Holmes and others had extended the Uranium Lead dating method into a Lead-Lead method called the Holmes-Houderman System that took into account Lead 204 the natural non-radiogenic lead isotope. This is the system that tells us the age of meteorites, the Earth, planets, and the entire solar system. So we'll take a minute to show how it works. The idea is to start with the Uranium to Lead growth equations and produce an equation that fits the definition for the slope of a line. We start by dividing both sides of the growth equation by the number of Lead 204 isotopes. We move the Lead term on the right side of the equation to the left side, leaving only Uranium on the right. We then divide the top equation by the bottom. This creates ratios on the left and right. We then replace the Uranium ratio on the right with the known value, 1 over 137.88, found in Allende and elsewhere. This knocks out the need to measure Uranium content altogether. If we graph this equation using the 207 to 204 ratio as the y-axis and the 206 to 204 ratio as the x-axis, we see that the left-hand side of our Holmes-Houderman's equation is the slope of a straight line. The right-hand side depends only on time. For any given time t, we'll have a straight line. Replacing the initial Lead ratios with the standard from Canyon Diablo, we see that all lines pass through this point. Now if we have t stand for the amount of time since the material formed, we can use the present day Lead ratios from the Allende CAI for the other point on the line. Other ratios from Allende CAIs also fall on this line. In fact, all the materials formed around the time these CAIs formed will fall on this line. That's why this isochron is called the geochron. It represents the age of the solar system's planetesimals building blocks. So now we have an equation for this earliest time t. It's a transcendental equation, it cannot be solved algebraically, but computer iteration processing gets us as close as we want. The solution gives us t equal to 4.567 billion years plus or minus 70 million years. This is the oldest age of all meteorites, meteors, asteroids, comets, moons, planets, including the Earth, as well as the Sun. In fact, whenever you hear that the Sun is 4.5 billion years old, that number came from this Holmes-Houderman's process for radiometric rock dating.